Lightningposition in storm may circle strongest updrafts

Lightning position in storm
may circle strongest updrafts

New finding could help in predicting hail, tornadoes

One of a series of stories covering the
quadrennial International Conference on Atmospheric Electricity,
June 7-11, 1999, in Guntersville, Ala.

June 11, 1999: While lightning is one of the most
dangerous aspects of severe storms, it appears that inside the
storm lightning itself avoids the most violent regions. This
finding has implications for tracking where tornadoes are likely
to form and where large hail may fall.

"This can be a monitor of severe storm intensity, another
tool to monitor when storms might produce tornadoes or hail,"
said James Dye, a researcher at the National Center for Atmospheric
Research in Boulder, Colorado. Dye spoke this week at the International
Conference on Atmospheric Electricity.

Lightning has long been associated with convection
in storms. At a simple level, the charging mechanism is similar
to shuffling across the carpet on a cold winter day, then touching
a door knob. The friction between your feet and the rug builds
a static electric charge that you carry until you touch something
grounded. Inside thunderstorms, the mechanism which separates
charges is collisions between growing snow pellets and numerous
ice particles.

But storms still conceal a number of
surprises in many elaborations and variations on that simple
description. Dye said that scientists now are learning that the
most energetic storms are prolific sources of intracloud lightning.
Intracloud lightning, though obscured by the clouds, mayÂ provide
another tool forÂ monitoring severe storms.

Dye's findings come from the Stratospheric-Tropospheric Experiment:
Radiation, Aerosols and Ozone (STERAO) Deep Convection experiment
in northeast Colorado in June and July 1996. He used data from
a French radio interferometer that collected five hours of uninterrupted
observations of the locations of lightning within two major electrical
storms, and from Doppler radar that shows wind direction and
speed within a storm. Combining the two data sets showed where
lightning was relative to updrafts and downdrafts.

Left, Above:The main reflector for a Doppler
radar, one of the principal tools for peering inside severe storms.
(NSSL/NOAA)

"It seems that lightning channels themselves are not
in the most intense updrafts," Dye explained, "but
in the weaker updrafts and downdrafts." This would seem
to go against expectation. But while the lightning avoided the
updraft cores, it became more frequent around the cores as the
storm grew stronger.

Dye said that storms with updrafts at speeds less than
5 meters/second (about 11 mph) produce little or no lightning.
Storms with updrafts of 10 to 20 meters/second (22 to 44 mph)
might have flash rates of 5 to 20 strikes per minute. At more
than 40 meters/second (90 mph or more), things get busy and the
flash rate goes to 1 per second or more.

Dye says no one knows sure why, but the answer probably lies
in the microphysics of ice and hail formation, the separation
of charges as these bodies grow and move past each other, and
how they are transported inside the storms by vertical and horizontal
winds.

The French interferometer is one of several systems that
have been developed in recent years to give scientists an inside
view of lightning in storms. Cloud-to-ground strikes across the
United States can be recorded by the National Lightning Detection
Network. But intracloud strikes in local storms can be recorded
only by the Lightning Detection and Ranging system at Kennedy
Space Center and the Lightning Mapping System at the New Mexico
Institute of Mining and Technology. Also, when they are overhead
at the right time the Lightning Imaging Sensor and the Optical
Transient Detector, both in orbit, can count lightning from above
the clouds.

An exciting addition to this new suite of research tools would
be a satellite-borne Lightning Mapping Sensor, under study at
the Global Hydrology and Climate Center, that would observe the
Earth continually from geostationary orbit.

"This would provide much more information in terms of
intracloud strikes," he said. "It could be an additional
forecasting and nowcasting tool" for meteorologists watching
severe storms.

Dye cautioned that his and other results are not conclusive
yet, but "they're highly suggestive and promising, but we
have more work to do."